1. Field of the Invention
The present invention relates to a hot tandem rolling mill which can perform both shape control by shifting rolls and wear dispersion.
2. Related Background Art
A roll-shift type rolling mill schematically shown in FIG. 1 (for example, U.S. Pat. No. 5,640,866, Japanese Patent Laid-Open No. 7-232202, and the like) is known as a roll mill used to obtain a very flat plate product. According to the roll-shift type rolling mill, upper and lower work rolls 1 having convex and concave parts as shown in U.S. Pat. No. 5,640,866 are alternately shifted in the axial direction of the rolls, and roll a plate member 2, thereby controlling the shape in cross section of the plate member.
On the other hand, in an ordinary hot finishing rolling mill, a rolling part of the surface of a roll is worn as the number of plates which are rolled increases. Especially, the end parts of the roll are worn more than the center part. When a plate to be rolled next is wide, the cross section of the plate has a shape in which the both ends are thicker (those are called cat's ears) as schematically shown in FIG. 2 and the rolled shape deteriorates.
In a conventional hot finishing rolling mill, therefore, the width of a plate cannot be freely changed and rolling operation is performed according to a predetermined operation schedule (called a coffin schedule) as shown in an example of FIG. 3. According to the example of FIG. 3, the axis of abscissa denotes the number of rolling coils after roll replacement and the axis of ordinate shows the rolling width of plates. In order to obtain a necessary plate width, rolling operation for narrow plates is repeated and the rolls are warmed, and plate members are rolled in accordance with the order of the wider plate members. Other various operation schedules are used according to hot finishing rolling lines.
In the conventional hot finishing rolling mill, as mentioned above, since the rolling operation is performed according to a predetermined operation schedule, there is a problem that schedule-free rolling operation which is performed irrespective of the plate width cannot be executed.
Consequently, there has been proposed that the upper and lower work rolls are axially shifted relative to each other by using the above-mentioned roll-shift type rolling mill, thereby dispersing local wear of the surface of the roll and achieving the schedule-free rolling operation. In this case, however, since a shifting function is used to disperse the wear of the surface of the roll, there is a problem that an inherent control of the cross section shape cannot be performed.
That is, hitherto, the function of shifting the upper and lower work rolls of the roll-shift type rolling mill in the axial direction is used to control the cross section shape of a steel plate. The shifting function is, however, used for dispersing the wear of the roll, so that the shape cannot be controlled. There is consequently a problem that the wear dispersion and the cross section shape control are incompatible.